iio:magnetometer:ak8974: Fix alignment and data leak issues

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.
As Lars also noted this anti pattern can involve a leak of data to
userspace and that indeed can happen here.  We close both issues by
moving to a suitable structure in the iio_priv() data.

This data is allocated with kzalloc so no data can leak appart from
previous readings.

Fixes: 7c94a8b2 ("iio: magn: add a driver for AK8974")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: <Stable@vger.kernel.org>

drivers/iio/magnetometer/ak8974.c

@@ -192,6 +192,11 @@ struct ak8974 {
 	bool drdy_irq;
 	struct completion drdy_complete;
 	bool drdy_active_low;
+	/* Ensure timestamp is naturally aligned */
+	struct {
+		__le16 channels[3];
+		s64 ts __aligned(8);
+	} scan;
 };
 
 static const char ak8974_reg_avdd[] = "avdd";
@@ -657,7 +662,6 @@ static void ak8974_fill_buffer(struct iio_dev *indio_dev)
 {
 	struct ak8974 *ak8974 = iio_priv(indio_dev);
 	int ret;
-	__le16 hw_values[8]; /* Three axes + 64bit padding */
 
 	pm_runtime_get_sync(&ak8974->i2c->dev);
 	mutex_lock(&ak8974->lock);
@@ -667,13 +671,13 @@ static void ak8974_fill_buffer(struct iio_dev *indio_dev)
 		dev_err(&ak8974->i2c->dev, "error triggering measure\n");
 		goto out_unlock;
 	}
-	ret = ak8974_getresult(ak8974, hw_values);
+	ret = ak8974_getresult(ak8974, ak8974->scan.channels);
 	if (ret) {
 		dev_err(&ak8974->i2c->dev, "error getting measures\n");
 		goto out_unlock;
 	}
 
-	iio_push_to_buffers_with_timestamp(indio_dev, hw_values,
+	iio_push_to_buffers_with_timestamp(indio_dev, &ak8974->scan,
 					   iio_get_time_ns(indio_dev));
 
  out_unlock: